The effectiveness of a computer is often evaluated based on its processing speed of an instruction. A single-cycle instruction is an instruction that can be executed and completed within a cycle and in the mean time allows a microprocessor to pre-fetch the next instruction. However, not all instructions in a program are single cycle instructions. How to reduce the processing time of an instruction has been a great concern to the designers of computer processors.
When executing the general logic instructions of a program, a microprocessor is supposed to run an instruction and pre-fetch the next one totally in an instruction cycle by adding value 1 to a program counter (PC). The next instruction can thus be executed in the next cycle. Therefore, single cycle instructions can be executed consecutively. As shown in FIG. 1, while instruction N is executed instruction N+1 is also pre-fetched for execution in the next cycle. However, if an instruction executed is a “CALL” instruction, the pre-fetched instruction would not be executed in the next cycle because the “CALL” instruction is supposed to jump to a different address specified in the “CALL” instruction. To accomplish this instruction jump, the PC will be added with a discrete variable “M” specified In the “CALL” instruction instead of the usual 1 to make the PC value discontinuous. Before so doing, the program would need a no operation (NOP) instruction for loading the correct address of the variable “M” to the PC for fetching and executing the instruction called by the “CALL” instruction. The insertion of the no operation requires at least one more instruction cycle that usually deteriorates the microprocessor's effectiveness.